The present invention is concerned with oxidation and in particular the oxidation of saturated and unsaturated hydrocarbons using molten alkali metal and/or Group II metal carbonate. The invention may be used to oxidize alkanes to produce oxygen containing organic compounds such as alcohols, and carbonyl containing compounds such as aldehydes and ketones and carboxylic acids. In addition it may be used to oxidize alkenes to produce epoxides or aromatic substrates to their corresponding oxygenated species. In particular the invention is concerned with the oxidation of C1 to C4 alkanes and more particularly methane, ethane, propane and butane, preferably derived from natural gas.
The availability of large quantities of natural gas has led to numerous proposals to upgrade the gas to more valuable chemicals including oxygen-containing organic compounds. The only commercial processes so far developed have required the sequential steps of catalytic dehydrogenation of the alkane to form an olefin followed by oxidation of the reactive olefinic site to produce the oxygenate. Other processes have been proposed for the direct oxidation of alkanes to oxygenates. However, these processes have low conversion and low selectivity and usually require high temperatures.
The two stage vapour phase oxidation of propylene to produce acrylic acid is known. However, there is no known commercial process based on propane oxidation to acrylic acid. The production of acrylic acid from propane would be more attractive because of the significant price difference between propane and propylene.
It is known that molten metal salts can be used as oxidation catalysts. For example U.S. Pat. No. 4,785,123 proposes the use of molten nitrates for the oxidation of alkylenes and alkanes. The oxidation of propylene and isobutene using a mixture of molten potassium nitrate and molten sodium nitrate is exemplified although the yields of the oxidation products are low. Similar reactions are disclosed in U.S. Pat. Nos. 4,882,443; 4,883,889; 4,885,374; 4,943,643; 4,959,486; 4,992,567. Various co-catalysts are also proposed in European Patent Publication EP 0 268 870 and PCT Publication WO 92/09588 and these are said to improve conversion and selectivity.
A possible reaction mechanism for these premises is provided in Applied Catalysis A general 196(2000) 217224 entitled “The direct epoxidation of propene by metal salts” by T A Nijhuis, S Musch, M Mackhee and K A Monlijn.
Molten metal carbonates are also known for use in fuel cells and are described in a paper in the J Phys Chem B 2004 108 pages 7553 to 7556 by Chen et al. The paper is particularly concerned with determining the nature of the active oxygen species.
The present inventors recognize that that molten carbonate salts may be used as oxidation catalysts through the in situ generation of peroxo-carbonate species.